1. Field of the Invention
The present invention relates to optical scanning devices applicable to optical scanning devices for use in an electrophotographic image forming apparatus.
2. Description of the Related Art
In optical scanning apparatuses, a polygon mirror or a galvanometer mirror is used as a deflector for scanning with an optical beam. To achieve higher-resolution images and higher-speed printing, the rotation speed of the deflector has to be further increased. This poses problems regarding durability of a shaft, heating due to windage loss, noise, and others, thereby limiting high-speed scanning.
To get around these problems, deflecting devices using silicon micromachining has been studied in recent years. For example, as has been disclosed in Japanese Patent No. 2924200 and Japanese Patent No. 3011144, a scheme has been suggested in which a vibration mirror and a torsion bar that pivotally supports the vibration mirror are integrally formed with a Si substrate.
According to this scheme, the size of a mirror surface can be reduced. Furthermore, since reciprocating vibrations are caused by using resonance, low noise and low power consumption can be advantageously achieved even though high-speed operation is possible. Still further, with low vibrations and little heating, the thickness of a housing that accommodates the optical scanning device can be reduced. Therefore, there is also an advantage in which image quality is hardly affected even if a low-cost resin compound with a less composition ratio of glass fiber is used.
Japanese Patent Application Laid-Open No. 2004-279947 discloses an example of an optical scanning device in which a vibration mirror is used instead of a polygon mirror.
However, when a micromirror for sine-wave vibrations is used as a deflecting unit, the deflection angle is changed in a sine wave. Therefore, an fθ lens used for a present write optical system is used for a scanning image-formation optical system, the scanning speed is low in a peripheral image height, and therefore the scanning speed on the surface to be scanned is not uniform. If the uniformity of the scanning speed is poor, image distortion and other disadvantages may occur near a main scanning direction, thereby causing deterioration in image quality. Moreover, although Japanese Patent Application Laid-Open No. 2004-279947 also discloses an electrical correction of the speed uniformity, if a difference caused by this correction from the original is large, dots become sparse or concentrated stepwise, thereby changing density and causing an image deterioration. For this reason, the amount of electrical correction has a limitation. It has been revealed from a sensory evaluation that, if the difference is steep between adjacent areas, the difference can be detected as a density difference, and therefore the limitation of the amount of correction is determined accordingly. When a vibration mirror and an fθ lens that vibrate in a sine wave are used, the amount of correction required for uniform-speed scanning exceeds this limitation of the amount of correction, and therefore an electric correction cannot fully satisfy the need for correction.
To get around this problem, Japanese Patent Application Laid-Open No. 2005-215571 discloses the use of a scanning image-formation optical system having an image-formation characteristic (f·arcsin characteristic) as represented by:H=K×sin−1(θ/2θmax)where H is an image height, K is a proportionality factor, θ is a deflection angle, and θmax is an amplitude (maximum deflection angle). With this, an optical scanning device can be obtained that optically corrects the waist position of main-scanning ray bundle to achieve a wide effective write width and excellent scanning-speed uniformity.
However, if an optical correction is performed in the scanning image-formation optical system, a light beam enters a scanning image-formation optical element, in which the shape and refracting power is changed depending on the image height, at a different position with a different angle depending on the image height. Therefore, in particular, a deterioration occurs in wavefront of the light beam on the image-formation optical element in a peripheral image height, thereby disadvantageously causing a deterioration in beam-spot diameter and further causing a deterioration in image quality due to a wave optical aberration. Moreover, a deviation between image heights of a spot diameter of the main-scanning ray bundle on the surface to be scanned is large, thereby causing a deterioration in image quality.
Still further, Japanese Patent Application Laid-Open No. 8-286135 discloses an example in which a mechanism is provided that adjusts the positions of a light source or a coupling lens, and a cylindrical lens in an optical-axis direction to correct a beam-spot diameter on the surface of a photosensitive member. However, the number of such mechanisms that adjust the positions of the optical elements is two or more. Therefore, there are problems of less stability as a system, necessity of a scanning image-formation optical system of an fθ optical system even if an adjustment mechanism is provided, and inferiority to a vibration mirror in heating, vibration, noise, power consumption, and others due to the use of a polygon mirror.
Moreover, Japanese Patent Application Laid-Open No. H10-269599 discloses an optical pickup device in which it possible to reduce a focal distance of a variable focus lens part in a structure including the variable focus lens part and a fixed objective lens.